Ultrasonic flowmeter systems

ABSTRACT

Described are systems for measuring the rate of fluid flow wherein the time delay between acoustic pulses transmitted upstream and downstream in a fluid passing along a path of travel is multiplied by repeated transmissions in sing-around fashion. The systems of the invention have the advantage of fast response time, but do not require the measurement of very small time differences, particularly at low velocities, because of the cumulative effect of combing the time delays between upstream and downstream pulses over a period of time.

United States Patent McShane [451 Apr. 4, 1972 54] ULTRASONIC FLOWMETERSYSTEMS h liqlllilgfl lfi'i'lilflll'SQRfilflLlQAllQNS [72] Inventor:James L. McShane, Churchill, Pittsburgh, 198,000 3/1967 U.S.S.R ..73/194 A Pa. 146,517 8/1962 U.S.S.R ..73/l94 A [73] Assignee: WestinghouseElectric Corporation, Pitt- OTHER PUBLICATIONS sburgh, Pa. Improvementsin Sing-Around Velocity Meas. Forgacs, Jour- [221 Fllednai of Acoust.Soc. Am. 12/60 p. 1697 211 App]. No.: 17,196

Primary Examiner-Charles A. Ruehl Assistant Examiner-John Whalen 52 U.S.Cl. ..73/194 A isii Int. Cl. ..(;0111/00 Hem, and

58 Fi ld Se h ..73 194A 1 m l 57 ABSTRACT [56] References CitedDescribed are systems for measuring the rate of fluid flow UNITED STATESPATENTS wherein the time delay between acoustic pulses transmittedupstream and downstream in a fluid passing along a path of 3,329,0171967 YamamOlO et a1 A travel is multiplied by repeated transmissions insing-around 3,232,101 11/1966 Yamamoto 73/ 194 A fashion. The systems ofthe invention have the advantage of 31537309 11/1970 GFohegan 73/194 Afast response time, but do not require the measurement of 3,293,92112/1966 Rlordan at 73/194A very small time differences, particularly atlow velocities, :3 g if because of the cumulative effect of combing thetime delays l 05 yama at a between upstream and downstream pulses over aperiod of time.

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O I {B 275 BIB 4' SIB B REcEIvE I I I I I I -1= AI TIME NAI I- ATRANSMITI I I I I A RECEIVE I I I I I 53 BTRANSMIT I I I I I FIG.5B.

BREcEIv I I I I I E5 I XI X2 I I I T|ME mm I- lNVENTOR James L. McShoneSZAMAM ATTORNEY MIEIITEDIPR 41912 SHEET 3 [IF 5 wzrrkmm ULTRASONICFLOWMETER SYSTEMS BACKGROUND OF THE INVENTION The usual principle ofultrasonic flowmeters involves the transmission of ultrasonic wavesthrough a fluid medium in two directions, one upstream and the otherdownstream of the direction of flow, and comparing the transit times,normally over paths of equal lengths. The speed of propagation of thewaves in the fluid medium is the same over both paths and the transittime varies according to the velocity of the fluid medium which shortensthe transit time over the downstream path and lengthens the transit timeover the upstream path. From the difference between the upstream anddownstream transit times, the flow velocity of the liquid medium can becalculated by either a time difference or frequency difference techniqueThe basic theory of the time difference and frequency differencetechniques can be explained as follows: Assume, for example, that aconduit having a uniform flow of fluid of velocity, v, contains two setsof transducers facing each other at a spacing, d, between the two. Ifthe sound velocity in the fluid at rest is c, then the respectivetransit times downstream, 1 and upstream, i can be represented asfollows:

I d/H-v and t d/c-v If a pulse is simultaneously transmitted in bothpaths, the received signals arrive at times differing by A: 2,, z(2a'v/c v The velocity of the fluid, v, is almost always much smallerthan the sound velocity, 0, in the fluid at rest for all practicalapplications in liquids. For these applications, the approximateequation At 2dv/c is sufficiently accurate. Thus, A! is proportional tov with the calibration constant being a function of c, and v can bedetermined from the relationship:

v l/2d)c At where 1/2d) is a constant. Correction for changes in must bemade in accurate systems.

In the usual frequency difference technique, the received pulse in eachpath is used to trigger another transmit pulse, thus generating a trainof pulses in each path whose period equals the transit time. This isknown as the sing-around method and the combination of circuitries,transducers, and transmission paths for each pulse train is referred toas a singaround loop. The repetition frequencies, f, for the downstreamcase and f for the upstream case, are:

f, 1/r (c+v/d) andf l/r,,= c-v/d and the frequency difference is:

f fi *fu 2"/11 Thus, the frequency difference is proportional to thevelocity of the fluid, v, with no dependence on c, the sound velocity inthe fluid at rest.

One important advantage of the time difference system as compared to thefrequency difference system is fast response, since a velocitydetermination can be made each transmission interval. However, in thepast, attempts to provide a time difference system have usually involvedthe measurement of the time difference between received pulses after asingle transmission. This meant that very small time differences had tobe measured, particularly at low velocities; and the system wasdependent upon a determination of c, the sound velocity in the fluid atrest. The frequency difference system provides a relatively easilymeasured quantity and freedom from effects of the velocity in the fluidat rest, but has a very slow response time because of the long countinginterval required to accurately determine a frequency difference for lowvelocities. Furthermore, the frequency difference system requires twopairs of transducers, to avoid the condition where a transducer tries totransmit and receive at the same time. The time difference system, onthe other hand, can be implemented with one pair of transducers byswitching the transducer function from transmit to receive after thepulses are transmitted.

SUMMARY OF THE INVENTION In accordance with the present invention, atime difference flowmeter system is provided wherein pulses aretransmitted in both directions, using either two pairs of transducers ora single pair. The received pulses in each path trigger transmit pulsesin their respective paths in sing-around fashion, as in the frequencydifference system. However, the repetition of pulses is not continuousas in the frequency difference system but terminates after a givennumber of transmissions in each direction. The time difference betweencorresponding received pulses in each path is measured. This timedifference is expanded over the original time difference; and theexpanded time difference is then measured to determine the rate of fluidflow.

In one embodiment of the invention, the time difference between the lastreceived pulse in one train of sing-around pulses and the correspondingpulse in the other train is measured. This expands the time differenceby a factor equal to the number of repetitions which occurred. Theexpanded time difference can be measured much more easily to a givenaccuracy than the time difference between the first set of receivedpulses.

In other embodiments of the invention, the time differences betweencorresponding pulses in each train are summed. This summation, which isstill proportional to the basic time difference between received pulses,provides greater sensitivity and can be measured to even greateraccuracies in determining rate of fluid flow.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic illustration of the basic transducer arrangementof an ultrasonic flowmeter;

FIG. 2 illustrates time relationships of the received and transmittedpulses for the usual time difference system;

FIG. 3 illustrates a more practical configuration of an ultrasonicflowmeter wherein the transducers are disposed in the walls of afluid-carrying conduit rather than being disposed within the conduititself;

FIG. 4 is a schematic block diagram of one embodiment of the presentinvention;

FIGS. 5A and 5B comprise time relationships of pulse trains illustratingthe operation of the system of FIG. 4;

FIG. 6 is a schematic circuit diagram of the basic form of thec-correction circuitry for the embodiment of the invention shown in FIG.4;

FIG. 7 is a graph of output voltage versus time illustrating theoperation of the circuit of FIG. 6;

FIG. 8 is a schematic block diagram of another embodiment of theinvention;

FIGS. 9A and 9B comprise waveforms illustrating the operation of thesystem of FIG. 8;

FIG. 10 is a schematic circuit diagram of the c-correction" circuitry ofthe system of FIG. 8;

FIG. 11 comprises waveforms illustrating the operation of the circuit ofFIG. 10;

FIG. 12 is a schematic block diagram of still another embodiment of theinvention; and

FIG. 13 comprises waveforms illustrating the operation of the system ofFIG. 12.

GENERAL DISCUSSION OF TIME DIFFERENCE VELOCITY MEASUREMENT Withreference now to the drawings, and particularly to FIG. 1, a conduit Kis shown having a fluid flowing therethrough in the direction of arrow Vat a velocity v. Disposed within the conduit K is a first ultrasonictransmitting transducer S and a receiving transducer R The ultrasonicpulses transmitted from sending transducer S to receiving transducer Rtherefore, have a speed equal to (0+v) where c is the sound velocity inthe fluid at rest and v, as mentioned above, is the velocity of thefluid flowing through the conduit K. A second sending transducer S isprovided along with a second receiving transducer R,,. In this case,however, the ultrasonic wave energy transmitted from transducer 5,, totransducer R is in a direction opposite to the direction of fluid flowthrough the conduit K. Consequently, the velocity of the wave energypassing from transducer S to transducer R is (c-v).

As was explained above, the respective transit times downstream, t andupstream, t,,, can be represented as:

.4 and s (2) If a pulse is simultaneously transmitted in both paths, thereceived acoustic signals arrive at times differing by:

At t t (2dv/c v 3 which, as was mentioned above, can be expressed forpractical applications in liquids, where v c, by the approximateequation:

At a 2dv/c (4) Thus, At is proportional to v with the calibrationconstant being a function of c; and under the conditions for which v c,v can be determined from the relationship:

v E (l/2d)c At where l/2d is a constant.

The time relationship of the transmitted and received pulses is shown inFIG. 2. The pulses from transducer S and 8,, are transmittedsimultaneously. However, the time t required for the pulse to travelfrom transducer 8 to transducer R is less than the time I to travelbetween transducer S and transducer R,, for the reason that the pulsefrom the latter sending transducer is traveling upstream. The differencein time between the received pulses is, therefore, At. Assuming that Atcan be determined and that c can also be determined, the velocity of thefluid passing through the conduit can be determined in accordance withequation (5) given above.

The method of determining involves measuring the transit times as thefollowing derivation will show. Equation (3) can be written as:

A! 2V/d (d /C V 6 From equations l and (2) we can obtain:

.4 h By substitution from equation (7) into equation (6),

A! A li Solving for v gives the result v (ti/2) /tin.) 9) Thus, v can beexpressed exactly (i.e., does not require that v 0) in terms ofmeasurable quantities At, t and t The approximate form of equation (9)corresponding to equation (5) ISI v E Atlz xp (l0) where t no-flowtransit time.

As was previously mentioned, systems have been devised in the past fordetermining the velocity of fluid flowing through a conduit by measuringAt, the time difference between a single pair of received pulses. Thistime difference, however, is generally very small and exceedinglydifficult to measure, meaning that the accuracy of such systems islimited. As will be seen, the present invention provides a means fordetermining velocity in accordance with equation (9) given above, butwherein the quantity At is expanded or made larger by repeatedtransmissions in sing-around fashion.

A practical application of the invention for conduits would not includetransducers disposed directly within the conduit as shown in FIG. 1;although for velocity measurements in a medium of large extent, such asthe ocean, probes could be arranged as in FIG. 1. Rather, thetransducers would be arranged in or through the walls of the conduitperhaps as shown in FIG. 3. In this case, equations corresponding toequations (3), (5) and (9) are:

A! (2Dv cot 6/c v cos 6?) v 2(1/20 cot 6) c At, (v c) v =(D/2 sin 0 cos0 (At/[4 where D diameter of conduit K and 0 angle between acousticpaths and conduit axis. It will be noted that equations (ll), (12) and(13) differ from equations (3), (5) and (9) only in the constants, whichdepend on the geometry. For purposes of simplicity, the description ofthe various embodiments of the invention will be made under theassumption that the receiving and transmitting transducers are parallelto the direction of fluid flow rather than at an angle thereto, it beingunderstood that it is necessary only to modify the equations given byusing different constants if the transmitting and receiving transducersare disposed in the walls of the conduit.

ULTRASONIC FLOWMETER BASED ON MEASUREMENT OF N A T One specificembodiment of the invention is shown in FIG. 4. Sing-around loop Aconsists of amplifier 1, trigger circuit 2, astable multivibrator 3,blocking oscillator 4, transducers S and R and the acoustic path throughthe fluid. The initial transmit pulse occurs coincidentally with theenergizing of the sing-around circuit A by a signal on lead 10A.Blocking oscillator 4 generates a pulse which shock excites S Thereceived signal at R, consists of a short burst of oscillations. Thissignal is amplified by amplifier 1, whose output causes trigger circuit2 to produce a trigger pulse at a predetermined portion of each receivedpulse, particularly the leading edge of the first or second half-cycle.This trigger signal causes astable multivibrator 3 to change states andthe resulting multivibrator output triggers blocking oscillator 4,providing the regenerative sing-around action. The astable multivibrator3 does not operate normally in the free-run mode but is synchronized atthe higher sing-around repetition rate by pulses from trigger circuit 2.Its free-run capability is useful for self-restarting in the event anacoustic pulse is blocked, and for circuit testing purposes in theabsence of a fluid path. Another function is provided by the astablemultivibrator 3 by means of the interval during which the multivibratorremains in the state triggered by the pulse from circuit 2. During thisinterval, the multivibrator 3 is insensitive to trigger pulses generatedby false signals at the amplifier input. One source of false signals isthe occurrence of transmit pulses in the other sing-around loop, whichin the two-transducer embodiment, are applied directly to the amplifierinput. Sing-around loop B operates in an identical way.

Each interval of limited duration sing-around operation is startedperiodically by timing circuit 9, which provides a pulse which opens Agate 10 and B gate 11. Multivibrators 3 and 7 are gated ON and, asmentioned above, the first transmit pulse is generated in each loop. Asthe sing-around loops continue to run, the multivibrator 3 output signalis fed to a preset counter 15 which counts each multivibrator 3 outputtransition which produces a blocking oscillator 4 output pulse. Theinitial transmit pulse is the A, pulse as the waveform in FIG. 5A shows,and the counter is preset so that the Nth pulse A causes counter 15 toproduce an output pulse, which closes A gate 10 and deenergizesmultivibrator 3 and hence loop A. The waveforms of FIG. 5 assume thattwo pairs of transducers are being used. The Nth transmit pulse occurs(the initial one being called the 0th), but the sing-around loop A doesnot respond to the resulting received pulse. Counter 15 output alsoopens gate 12 so that the next transition of multivibrator 7 whichcorresponds to the Nth transmit pulse in loop B (By) is passed throughthe gate 12, all previous output signals from multivibrator 7 beingblocked. The output of gate 12 closes B gate 1 1 and stops thesing-around action in loop B.

The counter 15 output pulse, i.e., the A pulse, is also fed to the N Atcircuit 14, switching ON its output to a specific voltage leveldetermined by the c-corrector" circuit 13 in a manner to be described.The output of gate 12, being the B,- pulse, is also fed to the N Atcircuit 14, switching OFF its output. Thus, the N At circuit 14 producesan output pulse of which is Nt,,-Nt N At. The flowmeter output is theaverage value of a repetitive series of N At pulses. It will beappreciated that since the pulses fed to circuit 14 must each resultfrom the Nth transmitted pulses, the cycle times for the individualloops must be such that one loop does not lead the other by more thanone full cycle.

The c-corrector circuit 13 has an output waveform which is proportionalto l/! during at least the interval in which the N At pulse may occur,considering possible variations in sound speed c. This waveform isstarted by the timing circuit at the instant the two sing-around loopsare started and t for the ccorrector waveform is measured from thatinstant.

The N At circuit output voltage level during the N At pulse is equal tothe instantaneous output of the c-corrector circuit 13. The followinganalysis shows that the output of the N At circuit 14 is proportional toAt/(t t as required by equation (9).

Let the output of the c-corrector circuit 13 be Ii /t where E is aconstant voltage. The N At pulse has a voltage level equal to thec-corrector output level and it begins at M and ends at Nt The N A!circuit 14 average output is:

& a l E t ,t, (& At E T At, l2 h s NT A H (14) where T is the repetitionperiod of the intervals of limited duration sing-around operation. Meansfor producing a c-corrector circuit 13 output that is proportional toIII will be described later.

Other N At circuit embodiments are subject to correction for 0 changesin a similar way. Two specific N At circuits are: a ramp generator whichreaches a peak voltage dependent on N At, the ramp slope being madeproportional to HF; and a combination of oscillator and gate whichprovides an output in the form of a count where the number of cycleswhich occur during the N A! interval are counted, the oscillatorfrequency being made proportional to llt As an example of a rampgenerator, consider a capacitance C being charged by a current equal to1 where 1 is a constant current. The peak capacitor voltage reachedafter being charged by this current during N A! is:

I A: (NC) t tB A similar result can be shown for the case where cyclesare counted.

The preceding description of the system of FIG. 4 dealt with the casewhere two pairs of transducers are used. The modifications required forone pair of transducers are indicated by dashed lines in FIG. 4, andwaveforms are shown in FIG. 5B. Transducers S R and S R,, are eachconnected to the output of one sing-around circuit and to the input ofthe other. The amplifier is designed to recover quickly from theoverload presented by the other loop transmit pulse, and the operationof multivibrators 3 and 7 as previously" described prevents falsetriggering in either loop.

To avoid interference between early pulses in the two loops, a delay 1'is introduced at two points in the block diagram (FIG. 4). Delay 16delays the starting of loop B by a time 1 after loop A starts. Delay 17inserts a compensating delay before the A pulse is fed to the N A!circuit 14. Delay 16 also causes the c-corrector waveform to start aftera time interval 1' following a timing pulse from timing circuit 9occurs, so that the waveform will have the correct value during the N Atpulse which is delayed by delays 16 and 17 as described. These delayswere shown as separate blocks for simplicity but it is desirable andpossible to use the same delay circuit for both delay functions.

The basic idea of the c-corrector circuit is to generate an outputvoltage which varies proportionally to c (more exactly, c v Neglectingfor a moment the small efiect of v on I x1 i the total time for pulsesto travel between transducers, this time tsp d/c. That is, the no-flowtravel time tsp is inversely proportional to c. Thus, if we generate avoltage proportional to .vr)

it will be proportional to c (see equation If we produce a voltage whichvaries as l/t where t is the time following a transmit pulse, and samplethe value of this waveform at time typ corresponding to the arrival ofthe received pulse, the output will be proportional to III,

and hence to 0 Sampling the waveform, which varies as llt over the N Atinterval gives a correction according to exact equation (9) rather thanapproximate equation (10), as has been shown.

The desired waveform is produced by approximating the l/t function withan exponential decay function which can be constructed of resistors andcapacitors. The approximation need be good only over the limited rangeof variation of I corresponding to changes in c of a given fluid duringoperation, as due to temperature. For example, c for liquid water variesfrom 4600 to 5100 feet per second as a function of temperature, which isa deviation of i 5.1 percent from a mean value of 4850 feet per second.The c-corrector could also account for the wider difierence in a 0between different fluids, but it is likely that a manual adjustment ofthe c-corrector range would be used to account for most of thedifference. In the above example, the c variation is i 10.4 percent.

FIG. 6 shows a circuit whose output voltage can match the desired l/tfunction with good accuracy over a range of c variation of 30 percent, awider range than is generally needed. As will be seen, the circuit ofFIG. 10, hereinafter described, is based on the same curve-matchingprinciple; but for practical reasons has an added feature, a delaybetween the transmit pulse and the start of the exponential decay. Theeffect of the delay is to start the exponential decay nearer in time tothe range of interest. An advantage is to reduce the initial voltagerequired to get a given voltage in the range of interest.

The c-corrector waveform can be generated for each transmission or foreach set of repetitions. In the first case the time of sampling is typ;in the second case it is Ntxp. Thus, the time scales of the waveformsare merely different by a factor of N.

The circuit of FIG. 6 includes a source of potential E adapted to beapplied through a gate circuit 54 across capacitor 56. In shunt with thecapacitor 56 is the series combination of a resistor 58 and a secondsource of potential E The voltage decay across the capacitor 56 and theresulting output voltage e, can be represented by the curve of FIG. 7which is represented by the equation:

K l/RC and We choose K K and K so that e, is proportional to (l/t) overa limited time range, this time range being represented as TD in FIG. 7.Therefore, by substituting various values of t in the range TD in theforegoing equation (16) and by following a curve-matching procedure, theoptimum values of E II, and RC can be determined.

The resulting output voltage e, proportional to 0 appears betweenterminals 60 and 62. Normally, the gate 54 is closed such that thecapacitor 56 is charged with the voltage E However, at time t a pulsefrom the timing circuit 9, for example, enables the gate 54 whereby thecapacitor 56 is caused to discharge through resistor 58 and the voltagesource E This continues until sampling of the output waveform occurs,whereupon the gate 54 is again closed, allowing capacitor 56 to rechargeprior to the start of the next measurement cycle. Better approximationsof the l]! waveform are possible with more c-corrector circuitelaboration (which would add more terms to equation 16)).

As will be appreciated, the system shown in FIG. 4 utilizes only the NAt time shift. It is, however, also possible to utilize (N-l) At, (N-2)At, and other time intervals in addition. Considering the output to bethe average voltage of a train of pulses, the output voltage can beincreased by including all pulses between the Mth and the Nth where M N.The output voltage will be given by:

where E is the pulse voltage level and T is the overall repetitionperiod. The simplest approach, one that provides the greatest outputvoltage is to include all pulses from At to N At i.e., M l The output,therefore, is equal to:

which is still proportional to At, but clearly larger than 2/ T) N A theoutput if only the N At pulse system were used.

A system for accomplishing the foregoing is shown in FIG. 8 whereinelements corresponding to those of FIG. 4 are identified by likereference numerals. The system again includes two sing-around loops Aand B each including an amplifier, a trigger circuit, an astablemultivibrator and a blocking oscillator.

The system of FIG. 8 operates in the same manner as the system of FIG. 4except for the readout and means of correction for speed of soundvariations. Again, the basic description covers the case where two pairsof transducers are used, this case being more straightforward. Changesrequired in the block diagram for the two-transducer case are againshown by dashed lines.

Timing circuit 9 periodically starts each series of repetitions of thesing-around loops by causing A gate 10 and B gate 11 to energizemultivibrators 3 and 7, respectively. A, and B, pulses (FIG. 9A) aregenerated at the instant the multivibrators are energized. When the A,-pulse is received by counter it produces a pulse which causes A gate 10to deenergize multivibrator 3 and hence loop A. The counter output pulseopens gate 12 so that the B pulse occurring later causes B gate 11 todeenergize multivibrator 6 and hence loop B. Thus, the timing and thedetermination of the duration of loop running times is the same as forthe basic multiple time difference system described with reference toFIG. 4.

In the system of FIG. 8, loop A pulses A turn ON flip-flop 68 andcorresponding loop B pulses turn it OFF. Thus, flipflop 68 generates atrain of pulses as shown in FIG. 9A whose widths are successively equalto the time between A and B between A and B and so forth, until, for thelast pulse, between A,- and B Pulses A, and B occur at the same time sono output pulse is generated. The same would be true for all pulses iffluid velocity, v 0. Flip-flop 68 output pulses have successive widthsAt, 2 At, N At, and the output voltage from average voltage sensingcircuit 72 would be as given by equation 18).

To provide correction for c variations, the voltage level E of theflip-flop 68 output pulses is varied as before by a voltage sampled froma waveform that varies as 1/t In this case, however, sampling by the NAt pulse to produce an output proportional to (At/t t is not practicalbecause the flowmeter output is made up of a series of pulses, which touse an extension of the c-correction method used previously, wouldrequire a corresponding series of waveforms proportional to 1/! withtime scales proportional to n, the number of the pulse (n 0, l, 2, ,N).One waveform varying as l/t could be used but it would be required to beaccurate over a wide t range. Also each pulse would have a difierentvoltage level, varying as UN (see eq. 14), which would weight earlypulses higher than later ones and thus reduce the advantage of timedifference expan- 5 sion. Instead, one waveform proportional to 1/! isused and it is sampled by a pulse of fixed length to produce a directcurrent voltage level E, which is fed to the flip-flop 68 to regulatethe level of the entire series of output pulses at a voltage E,

The use of a fixed sampling time of the l/t waveform is justified by theapproximate equations (5) and (10), which make the use of the fact thatv c in practical applications. Taking the instantaneous voltage of thel/t waveform at any instant between t, and I, would give a voltageproportional to c with good accuracy. A somewhat better approximation isactually indicated by the description of the c-corrector" circuit 66which follows later. First it should be pointed out that the l/twaveform is generated for each sing-around repetition rather than oncefor each series of repetitions as for the previous system. This merelyreduces the time scale by a factor of N and facilitates filtering thesampled pulses to produce a smooth direct current output E Thec-corrector 66 output, if a waveform E,/t is sampled for a duration t,and if the average value of a train of pulses having the sampled levelis used to determine E is:

If t, is made equal to the mean value of At expected over the velocityrange to be covered, then r, t, equals the mean value of t and E isproportional to l/(t t,,) to a good approximation. Thus, the output ofaverage voltage sensing circuit 72 is proportional to At/t t to a goodapproximation, as required.

For two-transducer operation, delay 16 is introduced as before to delaythe start of loop B by a time 1' following the start of loop A. Thisdelay adds a constant width to each of the flipflop 68 output pulses,successive pulses none having widths At 1, 2 At 1-, N At T. Tocompensate for this effect, a monostable multivibrator 70 is triggeredby each loop A pulse to produce a train of n pulses having width 1. Theaverage value of this waveform is determined by average voltage sensingcircuit 73, whose output is subtracted in subtracter circuit 75 toproduce an output proportional to At (for constant c). The c-correctorvoltage sets the output level of the monostable multivibrator 70 pulsesequal to that of the flipflop 68 pulses. The subtracter output voltageis where, as before, variations in E account for variations in c. Themodificatons can also be used with two pairs of transducers. Theyimprove the system in other ways: flip-flop 68 is not required togenerate very narrow or zero width pulses at any time, and thesubtractive output arrangement provides convenient method of zeroing themeter (i.e., by varying width or level of monostable multivibratorpulses) if residual At due to slight differences in the sing-aroundloops cause a zero offset.

The operation of the circuit of FIG. 8 with only a single set oftransducers can best be understood by reference to the waveforms of FIG.9B. In this case, only the transmitted pulses of waveforms A and B areemployed, the pulses in waveform B being delayed with respect to thosein waveform A, by the time delay 1. Since the flip-flop 68 is turned ONin response to a transmitted pulse in the A loop and turned OFF inresponse to a transmitted pulse in the 8 loop, its output appears aswaveform FF. The first pulse in waveform FF, for example. has a widthequal to r; the second pulse has a width equal to A! r, the third pulsehas a width equal to 2 Al r; and so on. The output of the monostablemultivibrator 70, shown as E2 N E EnAt. 2

The system of FIG. 4, if repetition period were T, output pulse levelwere 15,, and N 5, would have an average output voltage.

E,,= (5E At/T).

With the system of FIG. 8, on the other hand. the output voltage is:

Thus, the output voltage from the system of FIG. 8 is in creased by afactor of three over that for the system of FIG. 4. It is advantageousto make N as large as possible. Maximum N is limited by the N Atinterval approaching the sing-around period, t,,. In general, the outputvoltage is increased by a factor of The details of the c-correctorcircuit of FIG. 8 are shown in FIG. 10; and its operation is illustratedby the waveforms of FIG. 11. The operation of this circuit is similar inprinciple to that of FIG. 6, but differs in that a llt waveform isinitiated for each repetition of sing-around loop A. Furthermore, inorder to obtain the maximum output voltage, the correction voltage isgenerated by charging a capacitor during a sampling interval, t,. Thepeak voltage is held on the capacitor by providing a high resistance inthe discharge path. A long time constant is desirable to smooth rippleand to average out small fluctuations in the correction circuit output(see waveform PR in FIG. 11). Actual changes in the speed of sound willbe relatively slow and will be followed by the circuit. The resultingoutput voltage is applied to both the monostable multivibrator 70 andthe flip-flop circuit 68 to vary their output voltage levels inaccordance with changes in the speed of sound.

With specific reference to FIG. 10, transmitted pulses from loop A areapplied to a sample monostable multivibrator 76 which will produce, onlead 78, the pulses of waveform SMV of FIG. 1 1. These pulses occur eachtime a pulse is transmitted by loop A and have a width equal to t thesampling period. The trailing edge of each pulse in waveform SMVtriggers a second monostable multivibrator 80 which produces the outputwaveform DMV comprising pulses having a width t,,. The circuitry of FIG.10 includes a switch 82, an RC circuit 84, a sampler 86 and a peakreading circuit 88. The waveform SMV at the output of multivibrator 76on lead 78 is applied through resistor 90 to the base of transistor 92in the sampler circuit 86. The 0 voltage levels cut off the transistor92, thereby disconnecting the anode of a diode 99 in the peak readingcircuit 88 from near ground potential. As a result, the capacitor 96 inthe peak reading circuit is charged by the output from the RC circuit 84throughemitter follower 98 and resistor 100.

At the trailing end of each sampling pulse in waveform SMV, switch 82 isclosed (i.e., transistor 102 turned on) by waveform DMV, triggered asdescribed from waveform SMV. During the delay t thus initiated,capacitor 104 is charged and remains connected to 8+ through transistor102. At the end of the delay, transistor 102 is turned off bymultivibrator 80, and the voltage starts to decay in the RC circuit 84comprising capacitor 104 in parallel with resistor 106 and voltagesource 108. The advantage of delaying the start of the decaying portionof the waveform across the capacitor, waveform C in FIG. 11, is that ahigher voltage across the capacitor (output voltage) in the RC circuitcan be obtained with a small initial charge on the capacitor. In otherwords, only the portion of the (III waveform which is of interest'isgenerated. The output is derived from the emitter of an emitter followertransistor 110 in the peak reading circuit 88 and is used to clamp theoutput level of the flip-flop 68 and monostable multivibrator 70 shownin FIG. 8. The output of the sampler as applied to the capacitor 96 isillustrated by waveform S in FIG. 11; while the output of the peakreading circuit 88 is illustrated by the waveform PR in FIG. 11. It willbe noted in waveform C of FIG. 11 that the decaying portion terminatesat the end of the sampling pulse. If the sampling pulse occurs later dueto increased transit time in sing-around loop A, the capacitor voltagewill continue to fall as indicated by the dashed line, and a lowervoltage will be sampled.

ULTRASONIC FLOWMETER BASED ON PULSE SUMMATION WITH AUTOMATIC SPEED OFSOUND CORRECTION 4 gum 2 (2 equation (21) above can be written 2 N(N umT 2 (25) The fixed time interval during which the loops operate shouldbe chosen so that, for full scale velocity, coincidence between the Nthpulse in loop B and the (N+l )th pulse in loop A is closely approached.For this condition N E on h8 (26) N l t /t (21 where t,,, is the time inwhich the sing-around loops operate during each period T. Substitutingthese relationships in equation (25 z onuon A Ea 2 T 1,413 From equation(9),

AI/IAIB) 2v /d which can be substituted in equation (28) to yield E0 E 2on( on where the quantity in brackets is a constant. As exactcoincidence is approached between the A, and the B pulse, equations (28)and (29) approaches exact equality. In normal operation, deviations fromequality in these equations will occur because the loop running timecould end during the occurrence of an output pulse and because, forvelocities below full scale, coincidence as defined above will not beclosely approached. For large values of N, even these deviations aresmall; typically N I00 for practical liquid flow applications.

It can be shown that for N At t /2, i.e., half of the N At valuerequired for coincidence, which might correspond to a mid-scalevelocity, the percentage deviation from the ideal equation isapproximately l0O/2N, which, for N 100, is i- 0.5 percent variation. Thevariation stays within this limit over a wide range of c variation. Itshould be pointed out that since the above variations are based onmid-scale velocities, the variations as a percentage of full scale wouldbe reduced by half, or 0.25'percent.

A system in accordance with the principles of the inventionincorporating automatic speed of sound correction is shown in FIG. 12;while waveforms illustrating the operation of the circuit of FIG. 12 areshown in FIG. 13. The system again includes a pair of transducers S Rand R 8, disposed within a conduit, not shown, and provided with twosing-around loops A and B each including an amplifier, an astablemultivibrator and a blocking oscillator. The system is controlled by amaster multivibrator 112 having a period T illustrated by the waveformsof FIG. 13.At time t= 0 in FIG. 13, the master multivibrator 112 enablesthe astable multivibrator 18 in singaround loop A causing the firsttransmit pulse in loop A to occur at that instant. As soon as the firstpulse is transmitted in loop A, it is also applied to a delay monostablemultivibrator 116 which delays it by the time period 1' in FIG. 13. Thedelayed pulse, actually the trailing edge of the delay monostablemultivibrator 116 output pulse, triggers the flip-flop 114 which nowenables the astable multivibrator 26 in sing-around loop B, the firsttransmit pulse in loop B occurring at the instant multivibrator 26 isenabled. The result, of course, is that the pulses in loop A shown inFIG. 13 lead those in loop B, the first two pulses A and B, beingseparated by the time difference 1' The delayed A pulses continue to beapplied to the B-gate flip-flop 114, but after flip-flop 114 changesstate in response to the A,, pulse, it is insensitive to further pulsesuntil reset by the master multivibrator 112.

Master multivibrator 112 determines loop A running time r as well as theoverall period T between the start of each series of sing-aroundrepetitions. The two states of the output of master multivibrator 112, afree-running monostable multivibrator, have durations 1 and (T- t,,,,),respectively, both 7 of which are adjustable to establish 1,, and T. Toincrease the average output voltage, 2 should be a large portion of T,as shown in waveform V of FIG. 13. Accordingly, the dead time (T- t ismade only as long as necessary to reset the system for the next cycle ofrepetitions. Reducing dead time, (Tt,,,,), is desirable in that it alsoreduces output ripple and response time. The master multivibrator 112gates loop A ON directly through astable multivibrator 18 for interval teach period T. B-gate flip-flop 114 gates loop B ON in a similar mannerthrough astable multivibrator 26 during an interval (t -1) each periodT, B-gate flip-flop 114 being turned ON by delay monostablemultivibrator 116 and OFF by master multivibrator 112, the ON statebeing defined as the one which gates loop B ON.

The transmitted pulses from loop A are applied to one side of an outputflip-flop circuit 1 18; while the transmitted pulses in loop B areapplied to the other side of this same output flipflop 118. The resultis that theA pulses from loop A turn ON the flip-flop; whereas the Bpulses from loop B turn OFF the flip-flop, the resulting output waveformof the flip-flop being that shown in FIG. 13 as waveform FFl. The outputpulses from the flip-flop 118 are then applied to an average voltage,sensing circuit 120 which will produce an average output voltage equalto the average voltage of the flip-flop output waveform of FIG. 13. Thisvoltage, however, includes the time delay, 7, which is incorporated intoeach pulse at the output of flip-flop 118. Accordingly, the outputpulses from the monostable multivibrator 116, which are triggered by theA pulses, will have the waveform FF2 of FIG. 13. This waveform isapplied to an average voltage sensing circuit 123 and subtracted fromthe voltage from circuit 120 in subtracter 124 to derive an outputproportional to the velocity of the fluid passing between transducers SR, and R S,,,

Because the same delay circuit (multivibrator 116) generates theconstant portion of the width of FF 1 pulses and the width of F F2pulses, good zero stability in the output voltage results from thesubtraction of the average values of these two wavefonns. Voltage levelsof FF] and FF2 pulses should be nearly equal and derived from the samesource, for stability. Slight adjustment of the FF2 voltage level can beused as a zero adjustment to account for slight differences in rise andfall times between the FF 1 and FF2 waveforms and for slight residual Atresulting from different electronic delays in the sing-around circuits,or from path differences if two pairs of transducers are used. If twopairs of transducers are used, monostable multivibrator 116, B-gateflip-flop 114, average voltage sensing circuit 123, and subtracter 123,and subtracter 124 could be eliminated, the output being taken directlyfrom average voltage sensing circuit 120. However, to avoid having togenerate the narrow early pulses of width At, 2 Ar, etc., and to provideconvenient l7 zeroing as described, the arrangement of FIG. 12 hasadvantages for the case where two pairs of transducers are used.

The capability of this system to handle either direction of flow shouldbe pointed out. If two-way flow is expected, delay 1 can be made roughlyhalf the sing-around period. In the case of reverse flow, waveform FFlin FIG. 13 would show successively narrower pulses, the average value ofFF 1 would fall below the average value of FF2, and the output would benegative.

As will be appreciated, the sing-around loops in the embodimentsdescribed above operate for a limited time only. If loop running time isshort enough to prevent pulse coincidence in the two loops, a singlepair of transducers will sufiice as described. The ability to operatewith one pair of transducers can also be provided if a delay isintroduced in one loop to keep the upstream and the downstream pulsetrains interleaved. The length of the delay can be measured as theoutput or, if the delay is developed by a voltage-to-time convertercircuit, the controlling voltage can be the output. To avoid the verysmall delays required if correction is made in each transmission period,correction can be made after a given number of pulses has occurred, orafter a given amount of interloop shift has occurred. It may also beadvantageous to have a delay in each loop, one or both of which isadjusted to keep pulse trains interleaved, the output being proportionalto the differencebetween thetwo delays.

Although the invention has been shown in connection with certainspecific embodiments, it will be readily apparent to those skilled inthe art that various changes in form and arrangement of parts may bemade to suit requirements without departing from the spirit and scope ofthe invention.

I claim as my invention:

1. In a flowmeter, the combination of at least one pair of oppositelydisposed ultrasonic transducer means located in acoustic contact with afluid stream, a first feedback path coupling the output of a first ofsaid transducer means to the input of a second of the transducer meanssuch that signals transmitted upstream from said second transducer meansto said first transducer means are recirculated to the second transducermeans through said first feedback path, a second feedback path couplingthe output of said second transducer means to the input of said firsttransducermeans such that signals transmitted downstream from said firsttransducer means to the second transducer means are recirculated to thefirst transducer means through said second feedback path, means forcausing N pulses to be transmitted from both said first and secondtransducer means over a given time interval whereby each transmittedpulse will have a corresponding received pulse which is delayed withrespect to its transmitted pulse, and circuit means coupled to saidfeedback paths for generating a signal proportional to the time delaybetween only the Nth received pulses in the respective feedback pathsand for modifying said signal by an amount dependent upon the speed ofsound in said fluid at rest, said modified signal. providing anindication of fluid flow.

2. The combination of claim 1 wherein said modified signal isproportional to the quantity c N A t where N A t is the time differencebetween the Nth received pulses in the respective feedback paths and cis the speed of sound in said fluid at rest.

3. The combination of claim 2 including means for determining c from thetransit time of pulses between said transducer means.

4. The combination of claim 3 including means for generating a waveformand for producing an output which closely approximates the inverse ofthe square of time following a specific transmitted pulse, and means forsampling the instantaneous value of said waveform at a timecorresponding to a specific received pulse to generate a signal whichmodifies the determined rate of flow.

5. In a flowmeter, the combination of at least one pair of oppositelydisposed ultrasonic transducer means located in acoustic contact with afluid stream, a first feedback path coupling the output of a first ofsaid transducer means to the input of a second of the transducer meanssuch that signals transmitted upstream from said second transducer meansto said first transducer means are recirculated to the second transducermeans through said first feedback path, a second feedback path couplingthe output of said second transducer means to the input of said firsttransducer means such that signals transmitted downstream from saidfirst transducer means to the second transducer means are recirculatedto the first transducer means through said second feedback path, meansfor causing the same number N of pulses to be transmitted from both saidfirst and second transducer means over a time interval whereby eachtransmitted pulse will have a corresponding received pulse which isdelayed with respect to its transmitted pulse, and electrical circuitrycoupled to said feedback paths for generating a signal dependent uponthe speed of sound in said fluid at rest and proportional to Ar+2At+3At+...+NAt

where A l is the time difierence between the first received pulses ofsaid same number in the respective sing-around loops.

6. In a flowmeter, the combination of at least one pair of oppositelydisposed ultrasonic transducer means located in acoustic contact with afluid stream, a first feedbath path coupling the output of a first ofsaid transducer means to the input of a second of the transducer meanssuch that signals transmitted downstream from said first transducermeans to the second transducer means are recirculated to the firsttransducer means through said second feedback path, means for causingthe same number N of pulses to be transmitted from both said first andsecond transducer means over a time interval whereby each transmittedpulse will have a corresponding received pulse which is delayed withrespect to its transmitted pulse, the initial transmitted pulses in therespective feedback paths being delayed with respect to each other by atime delay 1-, and electrical circuitry coupled to said feedback pathsfor generating a signal dependent upon the speed of sound in said fluidat rest and proportional to r+(A t-i-r)+(2Az+1-)+(3Az+r)...+(NAt+1-),where OT is the time difference between the first received pulses ofsaid same number in the respective ring around UNITED STATES PATENTOFFICE @ERTIFICATE ()F CORRECTION lateni; N00 3 ,653 a 59 Dated April 1,1972 Inventor(s) James L. McShane It is certified that error appears inthe above-identified patent and that said Letters Iatent are herebycorrected as shown below: Column 1', line 25, change 0 d/c+v" to 0d/(c+v) and "t d/c-v to t d/(c-v) 2 2 line 28, change "(2 V/C V to 2dv/(c v line 37, change "v (1/2d)c At" to v 1 (c At) line 38, change(l/2d)" to l 2d line &9, change "(c+v/d)" to (c+v)/d and "c-v/d" to--(cv)/d Column 3: line 10, change "t (d/c+v)" to 13 d/(c+v) line 12,change "13 (d/c-v)" to t d/(c-v) line 15, change "('2dv/c -v to 2dv/(c-v 5 line 21, change "(l/2d) 0 At" to (c Ac) 1--;

line 25,- change "1/2df' to 2 a line ll, change "2v/d(d /c -v to (2v/c1)d /(c v 7 line 43, change 'Y(d /c V to d /(c v line H5, change "At (ZVtt /d)" to At ZVt t /d 5 line 52, change (dAt/2c to d At/(2t line 73,change "(2Avcot 9/c -(v cost (9) to 2Dv cost-9/[c (v cos 9) 5 line 7 4change (l/2D cot 9)c At, (v c)" to [l/(2D cot 9)]0 At, (v c) line 75,change "(D/2 sin 9 cos e)( At/t t to [D/(2 sin 9 cos QHAt/(t t Column U;line 1 1, change "NAt" to NAt 5 Column 6; line 53, change "K 8 K t K toK exp -K t)+K I I a FORM o-1o50 0 69) I USCOMM-PC QOSTG-POD v i 0.5.GOYIIKMINT PIIII TING OFII CI "Cl 0-!1-834 UNITED STATES PATENT OFFICE IPage 2 CERTIFICATE OF CORRECTION Patent No. 3, 53, 59 Dated April a 97Inventor(s) James L. McShane i It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 10; line 53; change "t T/t to (t 1" )/t Column 12, line 18,change At,2 l/3t, etc

to At, 2At, etc.

Column 1M, line 27, change "0T" to" A1:

Signed and sealed this let day of I January 19714..

(SEAL) Attest: v

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER v Attesting Officer r ActingCommissioner of Patents T FORM Po-mso (10-69)

1. In a flowmeter, the combination of at least one pair of oppositelydisposed ultrasonic transducer means located in acoustic contact with afluid stream, a first feedback path coupling the output of a first ofsaid transducer means to the input of a second of the transducer meanssuch that signals transmitted upstream from said second transducer meansto said first transducer means are recirculated to the second transducermeans through said first feedback path, a second feedback path couplingthe output of said second transducer means to the input of said firsttransducer means such that signals transmitted downstream from saidfirst transducer means to the second transducer means are recirculatedto the first transducer means through said second feedback path, meansfor causing N pulses to be transmitted from both said first and secondtransducer means over a given time interval whereby each transmittedpulse will have a corresponding received pulse which is delayed withrespect to its tranSmitted pulse, and circuit means coupled to saidfeedback paths for generating a signal proportional to the time delaybetween only the Nth received pulses in the respective feedback pathsand for modifying said signal by an amount dependent upon the speed ofsound in said fluid at rest, said modified signal providing anindication of fluid flow.
 2. The combination of claim 1 wherein saidmodified signal is proportional to the quantity c2N Delta t where NDelta t is the time difference between the Nth received pulses in therespective feedback paths and c is the speed of sound in said fluid atrest.
 3. The combination of claim 2 including means for determining c2from the transit time of pulses between said transducer means.
 4. Thecombination of claim 3 including means for generating a waveform and forproducing an output which closely approximates the inverse of the squareof time following a specific transmitted pulse, and means for samplingthe instantaneous value of said waveform at a time corresponding to aspecific received pulse to generate a signal which modifies thedetermined rate of flow.
 5. In a flowmeter, the combination of at leastone pair of oppositely disposed ultrasonic transducer means located inacoustic contact with a fluid stream, a first feedback path coupling theoutput of a first of said transducer means to the input of a second ofthe transducer means such that signals transmitted upstream from saidsecond transducer means to said first transducer means are recirculatedto the second transducer means through said first feedback path, asecond feedback path coupling the output of said second transducer meansto the input of said first transducer means such that signalstransmitted downstream from said first transducer means to the secondtransducer means are recirculated to the first transducer means throughsaid second feedback path, means for causing the same number N of pulsesto be transmitted from both said first and second transducer means overa time interval whereby each transmitted pulse will have a correspondingreceived pulse which is delayed with respect to its transmitted pulse,and electrical circuitry coupled to said feedback paths for generating asignal dependent upon the speed of sound in said fluid at rest andproportional to Delta t + 2 Delta t + 3 Delta t + . . . + N Delta twhere Delta t is the time difference between the first received pulsesof said same number in the respective sing-around loops.
 6. In aflowmeter, the combination of at least one pair of oppositely disposedultrasonic transducer means located in acoustic contact with a fluidstream, a first feedbath path coupling the output of a first of saidtransducer means to the input of a second of the transducer means suchthat signals transmitted downstream from said first transducer means tothe second transducer means are recirculated to the first transducermeans through said second feedback path, means for causing the samenumber N of pulses to be transmitted from both said first and secondtransducer means over a time interval whereby each transmitted pulsewill have a corresponding received pulse which is delayed with respectto its transmitted pulse, the initial transmitted pulses in therespective feedback paths being delayed with respect to each other by atime delay Tau , and electrical circuitry coupled to said feedback pathsfor generating a signal dependent upon the speed of sound in said fluidat rest and proportional to Tau + ( Delta t + Tau ) + (2 Delta t + Tau) + (3 Delta t + Tau ) . . . + (N Delta t + Tau ), where OT is the timedifference between the first received pulses of said same number in therespective ring around loops.